High power lasers are important for use in numerous industrial and military applications. Depending on the application, lasers can be operated in a continuous wave mode (CW) where the power output is essentially continuous over time or in a pulsed mode where the output takes the form of pulses of light of various durations and repetition rates. As more applications for lasers are found, there is a need to increase the power output of lasers. This has been limited by thermal effects which cause the laser beam quality to deteriorate. One way to increase the power output of a laser is to use solid-state lasers such as fiber lasers which are less prone to thermal effects. Fiber lasers that operate in both CW and pulsed modes have been developed, however short pulsed (<10 nsec) fiber array lasers have received less attention in the prior art.
Active fibers (glass fibers that have been doped with laser-active ions) offer the most electrically efficient, highest brightness laser source but are limited in the short pulse regime by non-linearities to approximately 250 KW peak power (250 μJ/pulse at 1 nsec) for conventional Large Mode Area (LMA) fibers and approximately 1 MW (1 mJ/pulse at 1 nsec) for Photonic Crystal Fibers (PCFs). When using active fibers, there are two approaches taken to scale the peak power handling. The first is to make even larger single mode fiber areas. This is difficult due to the index of refraction (<10−5) control necessary for single mode waveguide operation. The second approach is to combine multiple fibers into a fiber array. Techniques to combine multiple fiber outputs include:
Incoherent: the fibers are simply tiled side by side.
Spectral: Each fiber operates at a slightly different wavelength and the outputs are combined with a wavelength dispersive optic (such as a grating or prism).
Coherent: Each fiber is essentially an arm of a large interferometer and active phase control of each arm is necessary to match the phase fronts at the combination optic.
Actively stabilized coherent beam combination (CBC) of an array of fiber amplifier chains into a single coherent beam is a valuable method to multiply the output power or pulse energy by the number of amplifier chains. Prior art techniques only control the piston phase of pulsed fiber amplifiers, and thus, can only combine well matched amplifiers with less than approximately ⅛ wave of phase variation over the pulse. As a result, the output pulse energy from each pulsed fiber amplifier chain is limited to only a fraction of the stimulated-Raman-scattering (SRS) limited output. This type of system is disclosed in U.S. Pat. No. 7,502,395 issued May 10, 2009 and incorporated by reference.
Thus, a need exists for a method and apparatus that can phase fiber amplifiers with arbitrarily large phase variations up to coherent combining at the SRS limited output. This would be a factor of 10 increase in pulse energy. In addition, for some Lidar applications, extreme high contrast ratio (>100 dB) is required and a method and apparatus satisfying this criteria is needed.